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NBME 20 Answers

nbme20/Block 1/Question#16 (48.8 difficulty score)
A 30-year-old woman with multiple sclerosis ...

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submitted by thecatguy(14),

This is a very nitpicky question. As I see it, the 3 main concepts tested are:

  • This patient has trigeminal neuralgia (sharp, brief, episodic pain in the face), which is caused by a lesion to primary sensory fibers that carry pain sensation from the face.
  • Multiple sclerosis, which the patient has, is a demyelinating disease (i.e., it affects white matter).
  • The myelinated axons carrying pain sensation from the ipsilateral face enter the brainstem at the level of the pons and then descend (become the spinal tract of the trigeminal). These white matter fibers pass through the pons to synapse on the spinal nucleus of the trigeminal, which is in the medulla (nucleus = gray matter). (picture here). Therefore, a lesion in the white matter (i.e., plaque) in the pons could cause trigeminal neuralgia, and this phenomenon has been observed.

Above the level of the brainstem (thalamus & cerebral cortex), you have second order sensory neurons. Lesions in this part of the circuit are not generally in trigeminal neuralgia. I suppose they also want us to assume that once the spinal tract of the trigeminal enters the medulla, it's not myelinated anymore. I don't think this is completely true, but given the logic described above, pons would still be the better answer.

As people have pointed out, the primary sensory fibers carrying light touch sensation from the face synapse on the chief sensory nucleus in the pons immediately after they enter the pons. This question is not asking about those fibers though.

I got the question wrong too..

mightymito  Wow this is the best explanation yet! Thanks so much for very clearly walking us through a tricky question. +3  
lovebug  @thecatguy Are.... you a professor? thank you very much :) +  

submitted by whossayin(18),
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The mnincmeo I ekil rfo rmeminreegb hte locations fo eht acnalir vrnsee is eth 42,,2,4" ulr"e

eAobv st=ibmaren CN I + rbdIiMin =aI NC II,I I=VP nso CN V, ,IV I,IV eIdI Mla=IVlu NC XI, ,X XI, IIX

lovebug  @whossayin thanx so much!!! +  

submitted by sympathetikey(998),
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iPneirclp orneSys ulcesuN of eht iTngrmieal si ceadolt ni hte Pn,os as is eht roMot Tinigeraml Neclsuu fo eth sno.p ihsT naitrtoesnpe si aplyobrb iglaedn oemr iwht eth celrinpiP neSsoyr Nuscule.

submitted by strugglebus(153),
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hTe snpo has seevrn 8-5, so the rigiaemtnl dwuol eb cedeaftf eher

masonkingcobra  Thalamic pain syndrome would involve dysesthesias on the entire contralateral body so more than just the face. Also it occurs often after post-stroke. Additoinally, these dysesthesias appear weeks/months later +3  

submitted by feliperamirez(23),
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I do urednntdas atht eht rnapiclip luscune of het tngelmiria enrve si oltcdea at het psn.o tuB insec tsih eitpatn si anvhgi iltgrianme niaruagel dwul'nto oyu eb tidgrinups het ywptaah vnedviol in aipn dna tetrpa,umere hiwch in this acse dwluo eb eth nplias lecsuun eoald(ct in het ela)d?lmu

eBs,dsie I ofdnu isht at an erltica

A ecnrte ohhsptyise eritsutatb eht npia fo atrnieligm iargeaunl ot a trcanel ashmmeicn viiolnngv eht spar lisrao fo eht spnlai imagtenlir .]cusuenl5[


pg32  I literally had medulla selected the whole time and then changed it to pons simply because i felt the test writers were just seeing if we knew where the trigeminal nerve was located. bummer because I think your logic is way better. it's what i first thought when i read the case. +1  

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SM is a eaiyltmgenidn edsise.a st'Tha yhw I dnid't ckip tocerx ro mtlahua;s tyhe aer egyr trmea.t I kcedip dluelma euasecb I hte tienpoaminlisgr evnsre raesrevt hte leau.dlm I guess ew rea jtsu esoppuds to ueassm het aieltnnoideym si phnepigan in teh n?sop I otnd' ko.nw

woodenspooninmymouth  Sorry, I meant to say that the pons would have the cell bodies for the pain/temp neurons, no? +  

submitted by mamed(8),

Per boards and beyond, in the lateral pons is there is the spinal V nucleus which carries contralateral pain and temperature. Knowing that + CN V arises it in the pons, I went with pons. The only other option I had left not crossed out was cerebral hemisphere but that seemed to broad.

vivarin  BB also said not to use the rule of 4's to localize CN V to the pons so that's why I crossed it out... +  

submitted by chandlerbas(86),

theres an association made in 2017 with trigeminal neuralgia and MS - which is what i think this pt has. key thing to note is that while all motor and sensory fibers of CN 5 enter at the level fo the pons (however some also do enter at the level of medulla and even the sc via the spinal tract of 5 to synapse with the long sensory nucleus of 5) heres the link of trigeminal neuralgia and MS:

submitted by lilyo(53),

I was thinking along the lines of facial sensation which is mediated by the trigeminal nerve and the fact that the trigeminal nerve is located in the pons.